1. Field of the Invention
The present invention relates to a lawn mower, and more particularly to a powered lawn mower having a cutter blade rotatable in a substantially horizontal plane and a cutter housing which accommodates the cutter blade therein.
2. Description of the Prior Art
One known lawn mower, disclosed in U.S. Pat. No. 3,132,457, has a cutter housing which accommodates therein a cutter blade rotatable in a substantially horizontal plane. The cutter housing is partly defined by a circumferential wall that has a discharge duct defining a discharge port. U.S. Pat. No. 4,189,903 also shows a lawn mower having a cutter housing partly defined by an upper wall having a raised scroll extending fully along its entire circumference, the scroll having a discharge port. An annular inner member for closing the discharge port is detachably mounted in the scroll. When the inner member is mounted in the scroll, the cutter housing is fully closed along its circumferential wall, and grass can be cut into small clippings by a cutter blade housed in the cutter housing.
To use the cutter housing disclosed in U.S. Pat. No. 3,132,457 as a circumferentially fully closed cutter housing, it is possible to attach the annular inner member for closing the discharge pot so that grass can be cut into small clippings, as shown in U.S. Pat. No. 4,189,903. With the known cutter housing having a discharge port, generally, grass clippings are discharged laterally out of the cutter housing through the discharge port, or stored in a grass bag connected through a chute to the discharger port. It is preferable that grass clippings be further cut into smaller clippings before being discharged out of the cutter housing. In the case where the annular inner member disclosed in U.S. Pat. No. 4,189,903 is simply attached to the cutter housing, however, since the discharge port is closed by the annular inner member, it is impossible to further cut grass clippings into smaller clippings and discharge them out of the cutter housing.
U.S. Pat. No. 4,158,279 shows a cutter blade having cutting edges along its longitudinal marginal edges and air impellers disposed behind the cutting edges with respect to the direction in which the cutter blade rotates. The air impellers serve to generate air flows upwardly.
U.S. Pat. No. 4,189,903 also discloses a cutter blade having a cutting edge extending from an end thereof toward an intermediate portion near the center of rotation of the cutter blade. The cutter blade also has an air impeller disposed behind the cutting edge at the end of the cutter blade, and a lowering member on the intermediate portion for directing an air flow downwardly.
To cut grass clippings into smaller clippings, the air impeller at the end of the cutter blade for generating an upward air flow may be inclined toward the center of rotation of the cutter blade. If such an inclined air impeller were incorporated in the cutter blade with the lowering member disposed on the intermediate portion parallel to the cutting edge as disclosed in U.S. Pat. No. 4,189,903, then grass clippings would tend to flow upwardly due to the air flow produced by the air impeller inclined toward the center of rotation, and then drop onto the cutting edge on the intermediate portion of the cutting blade. However, a downwardly air flow produced by the lowering member parallel to the cutting edge would flow toward the end of the cutter blade under centrifugal forces, disturbing the air flow produced by the air impeller. Therefore, the grass clippings could not well be cut into small clippings, and could not well be distributed over the lawn.